Today, electrochromic devices are mainly commercially available as dimable mirrors in cars. However, in a near future electrochromic devices are expected to be widely used in a large variety of applications such as displays for computers and mobile phones, switchable windows, variable thermal emittance devices etc.
A typical electrochromic device is a thin film multilayer, comprising a cathode and an anode, separated by an ion conductor, and covered by transparent electron conductors on both outer sides. Such a thin film stack may be deposited on one substrate, if the ion conductor is a thin film (an all-thin-film device), or embedded between two substrates if the ion conductor is a polymer (laminated device). Such a device changes its optical transmittance from transparent to colored and back upon charge transfer between the anode and the cathode. It is required that both the anode and the cathode have suitable optical properties, i.e., are sufficiently transparent in the bleached state and sufficiently dark in the colored state. An extensive survey on electrochromic materials and devices can be found in C. G. Granqvist, Handbook of inorganic electrochromic materials, Elsevier, 1995.
In an electrochromic device—such as a window, a mirror, or a light reducing filter—it is desirable that the electrochromic thin film stack is not visually noticeable when it is in its bleached state. However, many known inorganic electrochromic oxide films, such as the oxides of nickel, iridium, and vanadium, have the drawback of not being perfectly colorless in their maximum transparent state; the films appear slightly yellow-green-brownish due to reduced transmittance (increased absorptance) in the wavelength range below 500 nm.
Examples of earlier work on increased transmittance in the bleached state are U.S. Pat. No. 5,798,860, concerning reduction of absorption in iridium oxide by addition of nitrogen, and U.S. Pat. No. 5,724,177, relating to increase of transmittance by applying an anti-reflecting layer on the outer side of the electrochromic device. For devices based on organic electrochromic materials, reduction of residual color in the bleached state is described in U.S. Pat. No. 6,188,505.